This invention relates generally to metallocenes, and more particularly relates to novel metallocene compounds, preferably chiral metallocenes, that are useful as catalysts, and to methods for synthesizing the novel metallocenes. The invention additionally relates to amino alcohol-derived ligands useful for preparing the novel metallocenes via metallation, and to methods of using the metallocene compounds, e.g., in catalysis.
Many processes and catalysts are known for the preparation of homopolymeric or copolymeric olefins and other polymers. Ziegler-Natta catalyst compositions, developed in the 1950s, were found to be particularly useful in the preparation of polyolefins. These catalyst compositions comprise transition metal compounds such as titanium tetrachloride and an alkylaluminum (e.g., triethylaluminum) cocatalyst. The systems were found to be advantageous because of their high activity, and were largely consumed during polymerization.
More recent catalyst systems for use in preparing polyolefins and other polymers are xe2x80x9cmetallocenes.xe2x80x9d The term xe2x80x9cmetallocenexe2x80x9d was initially coined in the early 1950s to refer to dicyclopentadienyliron, or xe2x80x9cferrocene,xe2x80x9d a structure in which an iron atom is contained between and associated with two parallel cyclopentadienyl groups. In general, the term is now used to refer to organometallic complexes in which a metal atom (not necessarily iron) is coordinated to at least one cyclopentadienyl ring ligand.
In contrast to the traditional Ziegler-Natta catalysts, metallocenes can provide a polymer composition containing a plurality of polymer molecules of substantially the same molecular structure. That is, if one high purity metallocene catalyst is used, the variance in the composition or molecular weight of the individual polymer molecules produced is minimal. With metallocenes, then, it is possible to control compositional distribution and other aspects of polymer molecular structure with unprecedented precision. Metallocene catalysts have other advantages as well. For example, metallocenes: (a) can polymerize almost any vinyl monomer irrespective of molecular weight or steric considerations; (b) provide the ability to control vinyl unsaturation in the polymers produced; (c) enable polymerization of a-olefins with very high stereoregularity to give isotactic or syndiotactic polymers; and (d) can function as hydrogenation catalysts for polymers as well as monomers. A. D. Horton, xe2x80x9cMetallocene Catalysis: Polymers by Design,xe2x80x9d Trends Polym. Sci. 2(5):158-166 (1994), provides an overview of metallocene catalysts and their advantages, and focuses on now-conventional complexes of Group IV transition metal complexes and cyclopentadienyl ligands (e.g., Cp2MX2, wherein Cp represents a cyclopentadienyl ligand, M is Zr, Hf or Ti, and X is Cl or CH3).
Metallocenes have also been found to be useful in catalyzing other types of reactions, i.e., reactions other than polymerization reactions. For example, metallocenes have been used as hydrogenation catalysts, dehydrocoupling catalysts, cyclization catalysts, substitution reaction catalysts, hydroformylation catalysts, carbomagnesation catalysts and hydrosilylation catalysts. See, e.g., Lu et al. (1997), Lanzhou Inst. Chem. Phys. 11(6):476-483; Halterman (1992), xe2x80x9cSynthesis and Applications of Chiral Cyclopentadienyl Complexes,xe2x80x9d Chem. Rev. 92:965-994; and Hoveyda et al. (1996), xe2x80x9cEnantioselective Cxe2x80x94C and Cxe2x80x94H Bond Formation Mediated or Catalyzed by Chiral Complexes of Titanium and Zirconium,xe2x80x9d Angew. Chem. 35:1262-1284. Thus, metallocenes are extremely versatile and valuable catalysts. However, prior metallocene catalysts have proved to be relatively difficult and time-consuming to synthesize, requiring expensive equipment, extreme reaction conditions, and multi-step processes that ultimately result in a low yield of the desired product.
Although a variety of organometallic catalysts have been discovered over the past 15 years, their discovery is a laborious process which consists of synthesizing individual potentially catalytic materials and subsequently screening them for catalytic activity. The development of a more efficient, economical and systematic approach for the synthesis of novel organometallic catalysts and for the screening of such catalysts for useful properties would represent a significant advance over the current state of the art.
Accordingly, there is a need in the art for new metallocene catalysts that can be synthesized without any of the aforementioned problems. That is, it would be desirable to have a simple, straightforward method for preparing chiral metallocenes that can be used in stereospecific catalysis, to be used in the stereospecific polymerization of olefins as well as in other stereospecific bond formation reactions. The present invention is addressed to the aforementioned needs in the art.
Accordingly, it is an object of the invention to provide a method for synthesizing metallocene compounds useful as catalysts, wherein the metallocenes can, if desired, be prepared as chiral compounds.
It is another object of the invention to provide such a method which involves synthesizing an amino alcohol-derived ligand from a silane reactant and an amino alcohol, and metallating the amino alcohol-derived ligand so prepared.
It is still another object of the invention to provide a method for synthesizing amino alcohol-derived ligands useful for providing metallocene compounds via a metallation reaction.
It is yet another object of the invention to provide novel metallocene compounds useful as catalysts.
It is an additional object of the invention to provide amino alcohol-derived ligands that may be used to prepare metallocene compounds via a metallation reaction.
It is a further object of the invention to provide a method for synthesizing polymers, particularly polyolefins, using the present metallocene compounds as polymerization catalysts.
It is a still further object of the invention to provide a method for making an array of amino alcohol-derived metal-ligand compound.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.
In one aspect of the invention, then, a method is provided for synthesizing a metallocene compound, the method comprising: (a) preparing an amino alcohol-derived ligand by reacting a silane reactant having two leaving groups with an amino alcohol in the presence of base; and (b) metallating the amino alcohol-derived ligand so prepared. In a preferred embodiment, the amino alcohol is a chiral compound, so that the metallocene ultimately prepared therefrom contains at least one asymmetric center, enabling catalysis of stereospecific reactions.
While the novel metallocenes may be used to catalyze any reactions for which metallocene catalysts are generally known to be useful, the present metallocene compounds are particularly useful as polymerization catalysts. One important application of the novel metallocenes is in catalyzing the polymerization of addition polymerizable monomers containing one or more degrees of unsaturation, to prepare polyolefins or other polymers. Another application of the present compounds is in catalyzing the polymerization of aromatic monomers which may or may not be addition polymerizable.
With respect to the preparation of polyolefins, such polymers, as is known in the art, can be prepared having a variety of steric configurations deriving from the manner in which each monomer is added to the growing polymer chain. Four basic configurations are commonly recognized for polyolefins: atactic, in which monomer orientation is random; isotactic, in which each monomer is incorporated into the polymer in the same configuration; syndiotactic, in which the configuration of monomers alternates along a polymer chain; and hemi-isotactic, in which unique and regularly repeating stereochemistries are present within a single polymer chain. The present metallocenes are useful for preparing polymers of desired tacticity, insofar as the chiral catalysts can be used to catalyze stereospecific polymerization. Generally, metallocene catalysts having C2 symmetry will give rise to isotactic polymers, while those catalysts having Cs symmetry will give rise to syndiotactic polymers.